Photosensitive lithographic printing plates conventionally comprise a photosensitive resin layer provided on a support having a hydrophilic surface. As for the plate-making method thereof, the photosensitive lithographic printing plate is ordinarily subjected to open frame exposure (mask exposure) via a lith film and then removal of the non-image area with a developer to obtain a desired printing plate. However, with the recent progress of digitized techniques, a computer-to-plate (CTP) technique of directly conducting exposure process on the surface of the plate by scanning according to digitized image data with highly convergent light, for example, a laser beam without using a lith film has been developed. Photosensitive lithographic printing plates adapted for the technique have also been developed.
As the photosensitive lithographic printing plates suitable for exposure with a laser beam, photosensitive lithographic printing plates having a polymerizable photosensitive layer are exemplified. Because the polymerizable photosensitive layer is easily enable to increase sensitivity by appropriately selecting a polymerization initiator or a polymerization initiation system (hereinafter also simply referred to as an “initiator” and an “initiation system”, respectively) in comparison with other conventional photosensitive layers.
However, when an image is drawn on such a photosensitive lithographic printing plate with a laser beam, a region of insufficient polymerization is formed in the edge of image due to an inadequate exposure amount depending on an energy distribution profile of the laser beam. Thus, sharpness of the edge of image is impaired, resulting in decrease in resolution. Also, in the region of insufficient polymerization formed in the edge of image, defect of removal occurs depending on alkali concentration of a developer or condition of a developing brush in a development processing step to cause a large variation of halftone dot area in a printing plate formed.
Moreover, since such a photosensitive lithographic printing plate comprises a support having a roughened surface according to electrolytic treatment or brush treatment in order to ensure hydrophilicity, image quality and sharpness are additionally injured and reproducibility in the shadow area is severely degraded due to scattering of reflection light at the laser exposure.
On the other hand, requirements for the high definition AM screen printing or FM screen printing have recently increased in the field of CTP technique. Therefore, the resolution of lithographic printing plate becomes an important performance.
The FM (Frequency Modulation) screen comprises fine halftone dots of approximately 20 microns irregularly arranged irrespective of screen angle and line number and expresses density gradation by halftone dot density per unit area. The features of FM screen print are that interference moire and rosette pattern do not occur, that tone jump in a halftone area of approximately 50% is avoided, and that due to the small size of halftone dot, overlap of halftone dots decreases so that the color reproduced can be brilliantly seen.
In contrast to the FM screen, the AM (Amplitude Modulation) screen comprises fine halftone dots regularly arranged at a certain angle and expresses density gradation by size of halftone dot per unit area. In Japan, a line number of the AM screen is ordinarily 175 lines per inch. On the other hand, printing using a screen line number of 200 or more is defined as the high definition AM screen printing.
The characteristics of high definition print include decreases in the moire and rosette pattern, improvement in texture of image and improvements in feeling of reality and reproducibility of detail.
However, photosensitive lithographic printing plates accompanying the degradation of reproducibility in the shadow area, for example, the photosensitive lithographic printing plates having a polymerizable photosensitive layer described above are difficult to use for the purpose of providing printing plates suitable for the high definition AM screen printing or FM screen printing, because they cannot reproduce extremely fine halftone dots.
In JP-A-2003-43703 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), a photosensitive lithographic printing plate comprising a support having provided thereon an intermediate layer containing a polymer compound including a constituent unit having a sulfonic acid group in its side chain and a photopolymerizable photosensitive layer in order is described. However, the photosensitive lithographic printing plate is still insufficient for a printing plate precursor-suitable for the high definition AM screen printing or FM screen printing. Particularly, since unevenness of halftone dot with the FM screen is severe, it is difficult to use the FM screen.